Transition metal surface compounds

Organosulfur Adsorbates on Metal and Semiconductor Surfaces. Sulfur compounds (qv) and selenium compounds (qv) have a strong affinity for transition metal surfaces (206—211). The number of reported surface-active organosulfur compounds that form monolayers on gold includes di- -alkyl sulfide (212,213), di- -alkyl disulfides (108), thiophenols (214,215), mercaptopyridines (216), mercaptoanilines (217), thiophenes (217), cysteines (218,219), xanthates (220), thiocarbaminates (220), thiocarbamates (221), thioureas (222), mercaptoimidazoles (223—225), and alkaneselenoles (226) (Fig. 11). However, the most studied, and probably most understood, SAM is that of alkanethiolates on Au(lll) surfaces. 

Gaussian-type orbitals, the computational requirements grow, in the limit, with the fourth power in the number of basis functions on the SCF level and with even a higher power for methods including correlation. Both the conceptual and the computational aspects prevent the computational study of important problems such as the chemistry of transition metal surfaces, interfaces, bulk compounds, and large molecular systems. 

Transition metal alkyl compounds react with the -OH groups on the surface of silica in a manner similar to that described for the silanol [reaction (13)] and as with the latter more than one type of bonding is possible. Silica dried at 200°C reacts with Zr(allyl)4 to give two molecules of propene per metal atom and utilizing in the course of this process two -OH groups per metal atom. The chemistry of the process is accurately described by the equation... 

The close structural similarity between metal clusters and elemental metals leads one to wonder at what size do metal clusters possess physicochemical properties generally associated with metals. Furthermore, given the fact that metal surfaces are important in catalysis, there is considerable interest in determining whether large transition metal clusters will be good models for chemical and physical phenomena at metal surfaces. The essential question, stated imprecisely, is how will increasing the metal-core size affect the electronic structure and reactivity patterns of transition metal cluster compounds ... 

To study the bonding in transition metal cluster compounds, a new type of Spherical Harmonic, with tensor properties, is required. This is because the metal d orbitals have (in addition to 1 o and 2 jt components) 2 8 components (d and dx2 y2) which are doubly noded in the plane perpendicular to the radial vector (see Fig. 16c) and which, therefore, behave as tensors. Two Tensor Surface Harmonic functions may be obtained from each Scalar Spherical Harmonic as follows146 ... 

The oxidation of small organic molecules has been extensively studied due to their relevance to electrocatalysis [88]. Among these molecules, methanol, formaldehyde, and formic acid have attracted the attention of the scientific community because of their simple structure and the relative simplicity of the investigation and interpretation of the results. There are several results which show that methanol on group VIE transition metal surfaces (Pt(l 11) [89], Pd(lll) [90], Pd(100) [91], Rh(lll) [92], and Ru(001) [93]) decomposes directly to adsorbed CO (or related compounds) and hydrogen atoms. 

The mechanism of trimerisation of acetylene on extended surfaces is thought to be similar to that observed in homogeneous catalysis using transition-metal cluster compounds.A stepwise mechanism, shown below is thought to occur ... 

Finally, as discussed in detail in the forthcoming chapters, LiFeP04, LiMnP04, and Li[MnFe]P04 compounds are much less reactive towards standard electrolyte solutions compared to Li ,MOj, compounds. It can be concluded that different surface chemistry is developed on Li-olivine and Li-transition metal oxide compounds. 

The final section in Chapter 2 deals with the molecular asp>ects of transition-metal catalysis. It serves as an introduction to Chapter 3. A characteristic feature of the transition-metal surfaces under catalytic conditions is their potential to restructure. Adsorbate overlayer adsorption can induce the surface to reconstruct with rapid diffusion of the metal as well as the overlayer atoms. The state of the surface may start to resemble that of a solid state compound. The state of the surface is not only strongly influenced by the composition of the reactant gas, but can also be strongly affected by the addition of promoters or other modifiers, that can result in alloy formation or new complex surface phases. 

Surface reconstruction is inherent to surface oxidation and sulfidation chemistry. In involves essentially surface corrosion and surface compound formation phenomena. The state of a surface can change from a metallic state to that of a solid oxide, sulfide, carbide or nitride depending upon the reaction environment. The surface of the epoxidation catalyst, discussed earlier, in the absence of Cl or Cs, for example, has a composition similar to AgO in the oxidizing reaction environment of the epoxidation system. The oxidation of CO over Ru can readily lead to the formation of surface RUO2 (see Chapter 5). In desulfurization reactions the transition-metal surface is converted to a sulfide form. The reactivity of the surface in these systems begins to look chemically more similar to that of coordination complexes. This we will illustrate in Chapter 5 for the C0S/M0S2 system. 

In order to study the stability of gold colloids in solutions of poorly interacting polymers, we chose block copolymers of styrene and ethyleneoxide (PS-b-PEO). Polyethyleneoxide does not interact strongly with transition metal surfaces and is thus a rather poor stabilizer for colloidal gold (5). It can, however, complex lithium cations and can also be protonated (9). In this way, PEO can bind compounds like LiAuCU and HAuCU, and we attempted to load inverse PS-b-PEO micelles by these gold salts. Subsequent reduction was expected to yield small gold crystallites entrapped in the micellar cores. 

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